package pen_exam.manbang2023_11_08;

import java.util.Scanner;

public class Class03_Replace {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String s = sc.nextLine();
        char[] str = s.toCharArray();
        int res = 0;
        for (int i = 0; i < str.length / 2; i++) {
            if(str[i] != str[i + (str.length /2)]){
                res++;
            }
        }
        System.out.println(res);
    }
}
